Initially the aliens wanted human DNA for their collection. Even space aliens everybody need a hobby.
Tricking numerous males to put sperm into prepared receptors seemed like a plan.
To get a feel for what would work, they uploaded human male preferences into their own mind states. Not knowing any better they decided to copy mind templates from artists depicting creatures similar to themselves. This gave their esthetic model a decidedly perverted slant.
With their new outlook the aliens found increased personal enjoyment in the collection process. Preparing the collectors is now an important artistic expression. The perverted mind states has become a widely downloaded resource on the galactic web.
The captives are rendered harmless by glueing their limbs together with self-repairing nano-goo. Being soft and squishy themselves the aliens have great respect for muscle and bone, not unlike what a human might feel about a sharp object near their eyeball. The forcefield powerlifting equipment can handle free-moving humans safely, but this technology is far from stealthy and is only used in far-off locations.
Prepared glue traps is a preferred capture method. With the victim fixed in place a custom wormhole portal can be generated within a day or two.
The aliens pride themselves of having a high moral standard. No death or harm of sentient beings is allowed. The receptor's bodies do not age. They are resistant to environmental hazards, supplied with nutrients and are able to regenerate any injury, just like the goo is. Everything is done to keep everybody involved as happy as possible.
Manipulation of earth money has proven to be useful.